This invention relates generally to apparatus for making printed circuit boards and, more particularly, to apparatus for maintaining precise registration between a circuit board and a printing mask containing the image of a conductive pattern to be printed on the circuit board.
It is known in the technology for fabricating printed circuit boards to utilize a mechanized circuit board printing device comprising a closed loop conveyor having a plurality of print frames disposed at regular intervals along it, a loading/unloading station where an unexposed, copper-plated and photoresist-coated circuit board is secured to the print frame which contains a printing mask for exposing a desired conductive circuit pattern on the circuit board photoresist and where an exposed circuit board is removed from the print frame, and an exposure station where a controlled light source forms a latent image of the conductive pattern of the printing mask on the circuit board photoresist. As this mechanized circuit board printing equipment is relatively expensive, it is desirable to obtain as high a throughput of exposed circuit boards as possible without jeopardizing the quality of the boards.
Regarding the aforementioned prior art mechanized printer, it has been found that disadvantages associated with the means for maintaining registration between the circuit board and the printing mask have been a major factor in contributing to a relatively low throughput value. In the past it has been customary to employ for this purpose either two locator pins situated at opposite ends of the print frame or four locator pins situated at each side of the frame. The locator pins pass through special bushings in the glass print frame and then pass through corresponding holes in the circuit board. The use of square locator pins has not been found to be practical, as square pins require corresponding square bushings in the glass artwork, which bushings are difficult to maintain to close tolerances and are expensive to manufacture. Moreover, square locator pins have a definite tendency to cut into and enlarge the holes in the circuit board, thereby introducing excessive play and registration error between the circuit board and the artwork.
The use of solid, round locator pins eliminates some of the disadvantages of square locator pins. However, the circuit board tends to become stuck on round locator pins because of the tighter tolerances associated with round pins as compared with square pins. As a result, excessive time is required for the operator located at the load/unload station of the mechanized printer to separate the exposed circuit board from the glass artwork. It is not uncommon for the operator to insert a screwdriver or other prying instrument between the circuit board and the artwork in an attempt to separate them, often with resultant damage to the relatively fragile glass artwork. If slightly undersized round pins are used in order to facilitate board insertion and removal, there is inevitably excessive play between the artwork and circuit board, resulting in an inferior quality printing operation.
The present invention is directed to an improved locator pin for maintaining accurate registration between the circuit board and the printing mask, in that it permits quick insertion and removal of the circuit board from its printing position adjacent the printing mask while maintaining precise alignment between the circuit board and printing mask. The result is a much higher printing throughput by the mechanized circuit board printer.